The absorbance measurement was carried out by a UV–visible
spectrophotometer (HP HEWLETT PACKARD 8453, UK) using a
1.0 cmpath length quartz cell. The modified surface morphology of
the paper-based device was characterized by scanning electron
microscopy (SEM) (JEOL, Ltd., Japan). Transmission electron
microscopy (TEM)was recorded by a H-7650 transmission electron
microscope (Hitachi Model, Japan). The levels of Cu2+ in real
samples were analyzed by inductively coupled plasma optical
emission spectrometry (ICP-OES) (CAP 6000 series ICP-OES,
Thermo Scientific, USA).